Update generate.txt

doc/doxygen/generate.txt

@@ -830,7 +830,7 @@ INPUT_ENCODING         = UTF-8
 # *.m, *.markdown, *.md, *.mm, *.dox, *.py, *.pyw, *.f90, *.f95, *.f03, *.f08,
 # *.f, *.for, *.tcl, *.vhd, *.vhdl, *.ucf, *.qsf and *.ice.
 
-FILE_PATTERNS          =
+FILE_PATTERNS          = *.h, *.hpp, *.h++
 
 # The RECURSIVE tag can be used to specify whether or not subdirectories should
 # be searched for input files as well.